I use a script I found on Spiceworks (edited slightly for functionality and logging) and I manually run the script (a VBS script executed via batch file) any time I do an e-cycling run and the PC's are physically off site and retired. I added the part where it creates a log file including date run so I could schedule it (if I choose to) and look at the log after the fact to see which assets have been removed.

Have a look if you like... I am not sure if you want to have those devices purged or not (but I do). If for some reason a system happens to be off for a month AND I run the script AND it gets purged mistakenly..... well the next scan brings it back just fine. This has only happened a couple of times for a volunteer system that had taken an extended holiday. I'll zip the batch file and VBS... I execute it with the batch and it will create the log file in the root of C: (change at your leisure).

*EDIT* I guess I should mention that it removes any non-manual entry that has not responded in 30 days or more..... (check the source and adjust to your needs)